Abstract: A relative humidity or water activity sensor includes a body defining a cavity with at least one resonant humidity sensing element comprising a crystal plate coated in part with an adsorbent material and at least one resonant temperature sensing element disposed inside the cavity. A membrane suitable for passing water vapor acts as a barrier to liquids and is situated between the cavity and the medium whose relative humidity and/or water activity is to be measured. The value of the relative humidity or of the water activity of the medium, and the temperature are determined from the resonant frequencies of the sensing elements. The invention also provides an installation including a plurality of such sensors.